1. Field of the Invention
The present invention relates to a battery charger and more particularly, to a battery charger for charging two or more rechargeable battery cells using a parallel battery charger topology which provides constant current charging.
2. Description of the Prior Art
Various portable devices and appliances are known to use multiple rechargeable battery cells, such as AA and AAA battery cells. In order to facilitate charging of the battery cells for such multiple cell appliances, multiple cell battery chargers have been developed. Both parallel and series topologies are known for such multiple cell battery chargers. For example, U.S. Pat. Nos. 5,821,733 and 6,580,249, as well as published U.S. Patent Application U.S. 2003/0160593, disclose multiple cell battery chargers configured in a series topology. U.S. Pat. Nos. 6,034,506 and 6,586,909 as well as published U.S. Patent Application U.S. 2003/0117109 A1 disclose battery chargers configured in a parallel topology.
In multiple cell battery chargers configured in a series topology, a series charging current is applied to a plurality of serially coupled battery cells. Because the internal resistance and charge on the individual cells may vary during charging, it is necessary with such battery chargers to monitor the voltage across and/or temperature of each cell in order to avoid overcharging any of the serially connected cells. In the event that an over-voltage condition is sensed, it is necessary to shunt charging current around the cell to prevent overcharging of any of the individual serially connected cells. Thus, such multiple cell battery chargers normally include a parallel shunt around each of the serially connected cells. As such, when a battery cell becomes fully charged, additional charging current is thus shunted around the cell to prevent overcharging and possible damage to the cell. In addition, it is necessary to prevent discharge of such serially connected battery cells when such cells are not being charged.
Various embodiments of a multiple cell battery charger configured with a serial charging topography are disclosed in the '733 patent. In one embodiment, a Zener diode is connected in parallel across each of the serially connected battery cells. The Zener diode is selected so that its breakdown voltage is essentially equivalent to the fully-charged voltage of the battery cell. Thus, when any of the cells become fully charged, the Zener diode conducts and shunts current around that cell to prevent further charging of the battery cell. Unfortunately, the Zener diode does not provide relatively accurate control of the switching voltage.
In an alternate embodiment of the battery charger disclosed in the '733 patent, a multiple cell battery charger with a series topology is disclosed in which a field effect transistors (FET) are used in place of the Zener diodes to shunt current around the battery cells. In that embodiment, the voltage across each of the serially connected cells is monitored. When the voltage measurements indicate that the cell is fully charged, the FET is turned on to shunt additional charging current around the fully charged cell. In order to prevent discharge of battery cells, isolation switches, formed from additional FETs, are used. These isolation switches simply disconnect the charging circuit from the individual battery cells during a condition when the cells are not being charged.
U.S. Pat. No. 6,580,249 and published U.S. Patent Application No. U.S. 2003/01605393 A1 also disclosed multiple cell battery chargers configured in a serial topology. The multiple cell battery chargers disclosed in these publications also include a shunt device, connected in parallel around each of the serially coupled battery cells. In these embodiments, FETs are used for the shunts. The FETs are under the control of a microprocessor. Essentially, the microprocessor monitors the voltage and temperature of each of the serially connected cells. When the microprocessor senses that the cell voltage or temperature of any cell is above a predetermined threshold indicative that the cell is fully charged, the microprocessor turns on the FET, thus shunting charging current around that particular battery cell. In order to prevent discharge of the serially connected cells when no power is applied to the battery charger, blocking devices, such as diodes, are used.
Although such multiple cell battery chargers configured in a series topology are able to simultaneously charge multiple battery cells without damage, such battery chargers are as discussed above, not without problems. For example, such multiple cell battery chargers require at least two active components, namely, either a Zener diode or a FET as a shunt and either a FET or diode for isolation to prevent discharge. The need for at least two active devices drives up the cost of such multiple battery cell chargers.
In order to avoid the problems associated with multiple cell series battery chargers, multiple cell battery chargers configured in a parallel topology are known to be used. Examples of such parallel chargers are disclosed in U.S. Pat. Nos. 6,034,506 and 6,586,909, as well as U.S. Published Patent Application No. U.S. 2003/0117109. U.S. Pat. No. 6,586,909 and published U.S. Application No. U.S. 2003/0117109 discloses a multiple cell battery chargers for use in charging industrial high capacity electrochemical batteries. These publications disclose the use of a transformer having a single primary and multiple balanced secondary windings that are magnetically coupled together by way of an induction core. Each battery cell is charged by way of a regulator, coupled to one of the multiple secondary windings. While such a configuration may be suitable for large industrial applications, it is practically not suitable for use in charging appliance size batteries, such as, AA and AAA batteries.
Finally, U.S. Pat. No. 6,034,506 discloses a multiple cell battery charger for charging multiple lithium ion cells in parallel. As shown best in FIG. 3 of the '506 patent, a plurality of serially connected lithium ion battery cells are connected together forming a module. Multiple modules are connected in series and in parallel as shown in FIG. 2 of the '506 patent. Three isolation devices are required for each cell making the topology disclosed in the '506 patent even more expensive to manufacture than the series battery chargers discussed above.
Another problem associated with parallel battery chargers is thermal runaway. In particular, it is known parallel battery chargers provide constant potential charging. With such constant potential charging, as the cell voltage increases, the temperature and charging current of the cell also increase. Continued constant potential charging of the battery cell causes the current to continue to rise as well as the rate of change of the temperature to increase significantly, resulting in a thermal runaway condition. Thus, there is a need for a battery charger which requires fewer active components than known battery chargers and is thus less expensive to manufacture and also avoids a thermal runaway condition.